Your enemy steps unwittingly into your trap, and you catch him by surprise with a sudden, paralyzing thrust.

Encounter   ✦     Martial, Weapon
Immediate Interrupt      Melee weapon

Trigger: An adjacent enemy shifts or moves

Target: The enemy

Attack: Strength vs. AC

Hit: 1[W] + Strength modifier damage, and the target is immobilized until the end of your next turn.

Published in Player's Handbook, page(s) 115.